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The British tourists who reportedly has a grenade thrown at them in Kenya were travelling in a van from Diani in Kwale to Mombasa when it was targeted as it slowed down, police told the Kenya-based Standard Digital.

"He vanished soon after he hurled the explosive," police chief Robert Kitur told the paper.

The grenade failed to explode and is being examined by police.

The attack comes on the same day Kenya celebrates 50 years of independence from British colonialist rule.

The Foreign Office said it is "urgently" looking into reports that a grenade has been thrown at a group of British tourists in Mombasa, Kenya.

"The grenade was detonated safely by experts," Mombasa police chief Robert Kitur said, according to Sky News. "We are investigating the incident and looking for the man who threw the grenade at the tourists and fled."